Solid waste has become a global problem that faces every major industrial nation, including the United States. The municipal and industrial solid waste stream is still largely disposed of in surface landfills that are rapidly diminishing in both volume and number, while the waste stream continues to grow. New incinerators and landfills are equally unpopular whenever they are proposed and ambitious recycling goals, particularly in large metropolitan areas, are not being reached. The crisis, projected for some time to be upon us by the mid-1990's, continues to approach and adequate solutions have yet to materialize.
Various methods are known for the stabilization and storage of solid waste materials. For example, U.S. Pat. No. 4,374,672 discloses a method of producing a stabilized fill material in water which comprises mixing cement (1-6% by weight) fly ash (45-80% by weight) and water (20-50% by weight) and placing said fill material produced directly in water while it is still in a flowable state.
U.S. Pat. No. 4,514,307 discloses a method for disposing of physically unstable, water containing, non-biologic organic waste material. The method comprises combining said waste material with cementitious reactants in the presence of water to form an environmentally acceptable, impermeable, load bearing material.
U.S. Pat. No. 4,576,513 describes a process for the terminal storage of pumpable wastes in salt caverns. The process comprises pumping a water-containing pumpable waste containing a liquid phase into a salt cavern and increasing the specific gravity of said liquid phase with a material selected from soluble salts which crystallize at cavern temperatures, organic materials which solidify at cavern temperature in the liquid phase of the pumpable waste or increase the specific gravity thereof, and adsorbents. The effect is to minimize the convergence of the salt cavern by narrowing the difference between the specific gravity of the salt cavern walls and the specific gravity of the liquid phase of the pumpable waste.
U.S. Pat. No. 4,577,999 discloses a process for storing liquid waste in salt cavities. In the process, the liquid waste with a pH of 7.0 or more is blended with additional materials to produce a pumpable mixture which has a boiling point above 85.degree. C. a flash point above 65.degree. C., vapor pressure at 60.degree. C. of up to 0.5kp/cm.sup.2, a viscosity of less than 300cP, and which forms no toxic or flammable gases. The pumpable mixture is then fed into the mine cavity and, after separation of the heavier and lighter liquid phases, both phases are separately pumped out of the cavern.
U.S. Pat. No. 4,692,061 relates to a process for dumping particulate solid waste materials in an underground salt cavern containing rock salt solution and equipped with pipelines for filling and evacuating said cavern. The process generally comprises pumping out as much of the rock salt solution from the cavity as possible; rendering the particulate solids dust-free with a dust suppressant; introducing the dust-free particulate solids into the cavity until said cavity is two thirds to about three quarters (3/4) full; and, solidifying any water present in the dust suppressant together with any rock salt solution remaining in the cavity, and sealing the cavity.
U.S. Pat. No. 4,917,733 discloses a pozzolanic mixture for stabilizing landfill leachate which comprises fly ash with an excess of lime, kiln dust and optionally bottom ash, which is combined with water to produce a stable cementitious pozzolanic mixture that hardens to a mortar-like materials.
None of the prior art references encountered, however, disclose or suggest an acceptable method for the permanent storage of solid waste in an empty and/or mined salt cavity.
Salt mining activities in bedded salt deposits create a vast underground network of rooms excavated in dry, impervious salt which is encased above and below by virtually impermeable shales. Thus, these empty salt cavities are perhaps the most environmentally isolated places available for the storage of waste materials with permanent isolation from the biosphere.
Salt mines in bedded salt deposits have historically been mined using the room and pillar method wherein pillars of salt are left in place to hold up the mine roof. Alternatively, cribbing or more expensive fill materials are placed in critical mine areas to artificially support the roof. In some areas of weak rock in the roof and/or because of overmining, all mining activities were simply abandoned. Backfilling with a structurally supporting material made from industrial solid waste offers an entirely new area of salt mine stabilization that has not been previously considered. Additionally, this backfilling technique can also provide a partial solution for the todays solid waste disposal problem.
Accordingly, it is an object of the invention to provide a method and pozzolanic mixture which chemically converts to a strong, hard durable mass in a salt environment with favorable leaching and stability characteristics.
A further object is to provide a method of backfilling salt mines with a structurally supporting material said pozzolanic mixture.
Another objective is to provide an environmentally attractive alternative to surface landfills for the permanent reuse/disposal of selected solid waste materials.